Raman art: Pigment revelations

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  • Published: Feb 1, 2016
  • Author: David Bradley
  • Channels: Raman
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Innovative naturalism

Giotto, Madonna and Child c. 1310/1315, Samuel H. Kress Collection, National Gallery of Art, Washington, D.C. (1939.1.256).

Florentine artist Giotto is well known for his innovative naturalism, his emotive faces and artistic skill in depicting space. Samples analysed with micro-Raman spectroscopy, scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX), polarized light microscopy, and electron backscatter diffraction (EBSD), have now revealed much about the blend of pigments he used in his art.

According to Barbara Berrie of the Scientific Research Department, at National Gallery of Art, in Washington, DC, USA, Giotto used inky washes under thin layers of egg tempera paint. Writing in the journal Heritage Science, she and colleagues Marco Leona of the Metropolitan Museum of Art, in New York and Richard McLaughlin of Oxford Instruments, Concord, Massachusetts explain that, "Yellow iron earth and lead tin yellow are present in the paint used to depict the lining of the Virgin’s mantle." The team used SEM-EDX on one of the yellow pigments to confirm that it is lead tin yellow type II, PbSn1-xSixO3. They also showed that the ratio of colourant to the glassy phase suggests that the material was produced for use as a pigment rather than as a glass. The blue pigment is azurite, the team reports, and contains no elemental impurities, although they also demonstrated that malachite and the rare green-blue mineral mixite, BiCu6(OH)6(AsO4)3(H2O)3 are also present in the blue.

Cool hues

The researchers concede that they cannot, from the evidence available, determine whether or not Giotto deliberately chose to use mixite as his green-blue pigment or whether this was simply a happy accident resulting from the choice of azurite rather than ultramarine. However, the mineral is rare so it is more likely that serendipity played a role. Of course, the team points out that the actual choice of colour, as opposed to mineral choice, would have been deliberate and the fact that the particular hue of mixite and the presence of the clear-hued lead tin yellow pigment suggest that he sought a cool hue rather than the warm blue of ultramarine. Ultramarine was quite typically used in other paintings of the Madonna during this period. "We have a very choice selection of the 'best of the best' from so long ago," Berrie told us. "It is true that he, at least in this beautiful work, he did not use the bluest blue of ultramarine, and the majority of the finest extant works do contain that precious colour." Whether or not that means he was bucking a trend is a different matter and a point that may never be addressed.

The team explains that their study used close visual stereomicroscopy of the surface of the painting and air-path X-ray fluorescence analysis (XRF) analysis. Cross sectional samples and paint scrapings from the bottom edge of the painting, or panel, were used for more detailed pigment characterization using a range of techniques, including micro-Raman spectroscopy. The XRF data could be ambiguous, hinting at nineteenth century pigments, but the Raman showed the samples to be different from those.

Painterly practice

A next step in the research on this artwork will be to investigate the geology of Europe and the mining history of the region in order to determine a putative source for Giotto's mixite. "The results of this work remind us that chemical analysis can add to our knowledge base of painting practice and materials, which helps to place artists’ works in a broader historical context, including comparison to the materials and methods used by their contemporaries and economic history. These findings add to our understanding of artistic innovation and a better appreciation of the nuance and worth of colour in painters’ practice," the team concludes.

Berrie told SpectroscopyNOW that, "Just today colleagues were in my lab suggesting trips to disused mines to discover the source of Giotto's blue and green pigments. That would be fun...but the next steps probably lie with historians of art, economics, technology, and science. They can use the information to underpin their research into the trade, manufacturing, and commerce of color in the trecento. Few documents survive on this, so chemical analysis plays a role in discovering more about their history."

Related Links

Heritage Sci 2016, 4, online: "Unusual pigments found in a painting by Giotto (c. 1266-1337) reveal diversity of materials used by medieval artists"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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